In this type power conversion system, an input voltage of the inverter contains rectification ripple caused by rectification of the converter, and thus there occurs a beat phenomenon that excessive current flows through the inverter and the load thereof and causes commutation failure or damage of the inverter. Various proposals have been hitherto made to suppress the beat phenomenon of the inverter as described above.
According to one of the proposals, a pulsation detecting unit for detecting a pulsating component at the DC side of the inverter which is caused by the rectification of the converter is provided, and the phase and frequency of the output voltage of the inverter are adjusted in accordance with the thus-detected pulsation (see Patent Document 1) .
In the power conversion system of the patent document 1, an inverter frequency correction amount is calculated on the basis of the pulsation rate of the detected DC input voltage, and the inverter frequency is adjusted in accordance with the pulsation of the DC input voltage, whereby current pulsation and torque pulsation can be reduced.
However, the method as described above is based on so-called feed-forward compensation of adjusting the inverter frequency in accordance with the pulsation of the DC input voltage, and this beat phenomenon occurs because an electric motor has a low impedance in frequency. Therefore, there is a case where torque pulsation may occur due to variation of a resistance value dependent on the individual difference of the electric motor or the temperature, and thus it is difficult to properly adjust the impedance. Furthermore, the pulsating component at the DC side is different in phase from the pulsating component at the AC side due to the efficiency or power factor of the power converter, and thus it is required to perform phase compensation by advancing the phase of the detected pulsating component or the like when the pulsating component at the DC side is used.
Furthermore, a power conversion system in which a state quantity corresponding to the torque of an electric motor is determined, the same frequency as the pulsation of the DC input voltage is integrated to generate a reference phase, and a sine-wave component and a cosine-wave component of the state quantity corresponding to the torque of the electric motor to the reference phase are calculated, thereby generating the output voltage phase angle compensation amount in accordance with the sine-wave component, the cosine-wave component and the reference phase is known in power conversion systems in which an output voltage phase angle is corrected on the basis of an output voltage phase angle compensation amount pulsating at the same frequency as pulsation of a DC input voltage input to an inverter, (see Patent Document 2).
However, the system of the patent document 2 detects phase current by a current detector and uses pulsation of Q-axis current, and thus it requires a component separation calculator containing a Sin-wave calculator, a Cos-wave calculator, a zero-cross detector, etc., plural integrators, a reference sine-wave calculator, etc. Therefore, a control system is very complicated. Accordingly, in order to implement this system, the capability of a microcomputer for processing software of the control system must be enhanced. In addition, with respect to the Sin-wave calculator, the Cos-wave calculator, the reference sine-wave calculator, etc., in order to implement sine wave accurately, it is required to shorten the operation time of the microcomputer, that is, high-speed processing is required. Therefore, it is required in the patent document 2 that the capacity of software increases and the microcomputer has high-speed performance, and thus there is a problem that the cost of the microcomputer for implementing the patent document 2 rises extremely.    Patent Document 1: JP-B-7-46918 (FIG. 1)    Patent Document 2: JP-A-2004-248450 (FIG. 1, FIG. 2, [0016] to [0018])